Estimating soil pCO2 using paleosol carbonates: implications for the relationship between primary productivity and faunal richness in ancient terrestrial ecosystems

In this paper we present a method for estimating soil pCO2 in ancient environments using the measured carbon-isotope values of pedogenic carbonates and plant-derived organic matter. The validity of soil pCO2 estimates proves to be highly dependent on the organic δ13C values used in the calculations. Organic matter should be sourced from the same paleosol profiles as sampled carbonates to yield the most reliable estimates of soil pCO2. In order to demonstrate the potential use of soil pCO2 estimates in paleoecological and paleoenvironmental studies, we compare samples from three Upper Jurassic localities. Soil pCO2 estimates, interpreted as a qualitative indicator of primary paleoproductivity, are used to rank the Late Jurassic terrestrial environments represented by the Morrison Formation in western North America, the informally named Lourinhã formation in Western Europe, and the Stanleyville Group in Central Africa. Because modern terrestrial environments show a positive correlation between primary productivity and faunal richness, a similar relationship is expected in ancient ecosystems. When the relative paleoproductivity levels inferred for each study area are compared with estimates of dinosaur generic richness, a positive correlation emerges. Both the Morrison and Lourinhã formations have high inferred productivity levels and high estimated faunal richness. In contrast, the Stanleyville Group appears to have had low primary productivity and low faunal richness. Paleoclimatic data available for each study area indicate that both productivity and faunal richness are positively linked to water availability, as observed in modern terrestrial ecosystems.

本研究提出一种基于成土碳酸盐（pedogenic carbonates）与植物源有机质的实测碳同位素值，估算古环境土壤二氧化碳分压（soil pCO₂）的方法。研究证实，土壤pCO₂估算结果的可靠性高度依赖于计算中所使用的有机质碳同位素比值（δ¹³C）数值。为获取最可靠的土壤pCO₂估算结果，有机质样品需与碳酸盐样品采自同一古土壤剖面（paleosol profiles）。为验证土壤pCO₂估算在古生态学与古环境学研究中的应用潜力，我们对三处晚侏罗世（Upper Jurassic）露头的样品开展了对比分析。以土壤pCO₂估算值作为古初级生产力的定性指示指标，我们对三个晚侏罗世陆相环境进行了排序：北美西部的莫里森组（Morrison Formation）、西欧非正式命名的卢林哈组（Lourinhã Formation），以及中非的斯坦利维尔群（Stanleyville Group）。现代陆相环境中，初级生产力与动物群丰富度呈正相关关系，因此推测古生态系统中亦存在类似关联。将各研究区推断的相对古生产力水平与恐龙属丰富度估算值进行对比后，二者呈现出显著的正相关关系。莫里森组与卢林哈组均表现出较高的古生产力推断值与动物群丰富度估算值；与之相反，斯坦利维尔群的初级生产力与动物群丰富度均处于较低水平。各研究区已有的古气候数据表明，古生产力与动物群丰富度均与水资源可利用性呈正相关，这一结果与现代陆相生态系统的观测规律一致。